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Andrew Blakeley-Smith MIRSE

Systems Consultant Teknis International Railroad Systems

"Signalling" to the average communications engineer normally means the process by means of which a telephone or data call is established, monitored and then cleared down between two or more parties. Any thought that there should be radical differences in procedures or basic philosophies that prevent or even make difficult this ojective is almost a contradiction of the requirement to communicate in the first place.

We thus now have a world wide communications network in which a telephone call can be placed anywhere and without too much difficulty, at least in principle, end up in the right place. This has been achieved by specifying the interface points of the various telephone networks, at a United Nations level, to be compatible in terms of signalling information, levels and frequency allocations etc. A spin-off from this has been that many of the indiv'dual items of hardware kave become interchangeable - even if the packaging is not identical and thus a walk through a telephone exchange will reveal a veritable United Nations of equipment happily CO-existing for most of the time.

Aircraft also need to communicate but unlike telephone exchanges also move around, thus when a piece of hardware fails a long way from home base, not only must the replacement meet the same electrical interfaces but it must also fit in the same hole as the failed equipment.

Thus from the Communications and aircraft industries has developed a modular way of specifying and building complex systems on a "Form Fit Function" basis. ATCS - the Advanced Train Control System - has grown out of the objectives of the use of the appropriate design philosophies to use widely available hardware elements to produce a signalling system which is multisourced and facilitates all operational and maintenance aspects of interworking between the railroads in the USA and Canada.

The specification documents for such a system are formidable and take up as much space as several telephone directories, it is not the intent of this paper to give a comprehensive summary of this documentation but to illustrate some of the more interesting elements in the design.